Science Magic – Creating a pH Rainbow For Any Age

So I showed this demonstration to my four year old and he loved it. I then showed it to my grade 10 science class, they loved it (although I couldn’t trick them to think it was actually magic) and then I showed my colleagues who were equally sceptical about its true magical properties but we’re non-the-less, still impressed.

So the science is pretty basic (excuse the pun). You use the pigment flavin that you extract from red cabbage (see below for a full how to) and see how it reacts with various substances. The only limitation is that the substance needs to be a liquid. When you mix your pigment (it’s actually called an indicator at this point) with your household substance, you get these fantastically colourful solutions which show how the pH’s differ. All you need is a few drops of your substance and however much indicator you desire. Red means your acidic, green your basic and blue your pretty neutral. It’s a fantastic way to introduce indicators/pH to your science class but it’s a also a great way to create that “Wow” factor and impress your kids. It also comes off as an amazing magic trick that will boggle the minds of your little ones.

How to present this to the young ones as magic:I had all my beakers lined up in a similar way to how it’s done above. I then asked my 4 year old which colour he’d like me to produce. Whichever colour he wanted, I was able to “pour it” because I knew which beaker to fill. He was thrilled.

How to utilize this as a teaching moment for middle and upper grades:
I set my demonstration up as shown above but I did so right under the projector screen. When I had poured all my colours, I simply posted this picture above it so the kids could compare the colour I had made with the pH they represent as well as common foods which fall under the range.

Click the picture for a larger version

I then explained the background of the red cabbage indicator – how the pigment responds to different levels of hydrogen ions. Also I went over a few of the other indicators we use in the lab (phenolphthalein, bromothymol blue, etc.)

How this demonstration can be used as a hands-on activity by your students:
Now I usually use the demonstration as a means of introducing pH and/or indicators to my students. Once I’ve done that, I get them to try. I’ll put them into groups and then let them make their own red cabbage indicator using the same method I used (only in much smaller quantities) in beakers labelled with their group members names. I then have the students use their indicator in my pH of household products activity lab. If you are interested in downloading a free copy of the lab, simply click here –> pH and Neutralization Lab <– and the download will start automatically.

How to make your red cabbage indicator:
1) Purchase a red cabbage from any grocery store ($2 – $3)
2) Rip the leaves into small pieces (no more than 2″ x 2″) and place them into a container
3) Pour boiling water over the leaves so that it at least covers them. Feel free to double the amount of boiling water, it wont make a difference and you’ll get a lot more indicator.
4) Let it stand for at least 10 minutes
5) Remove the cabbage leaves from the container and your indicator is ready

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To take this another step, I have my students create the ph color scale by also using pH paper as the test then after collecting the ph and indicator color of 9-14 liquids they use colored pencils to create their scale. The next day I give them unknowns and they have to estimate the ph using the indicator and the scale they created